1. Field of the Invention
This invention relates to a lithium ion secondary battery comprising a positive electrode and a negative electrode facing each other via a separator holding an electrolytic solution. More particularly, it relates to a battery structure providing improved electrical connections between each of a positive electrode and a negative electrode and a separator so that a battery may have an arbitrary shape, such as a thin shape.
2. Discussion of the Background
There has been an eager demand for reduction in size and weight of portable electronic equipment, and the realization relies heavily on improvement of battery performance. To meet the demand, development and improvement of batteries from various aspects have been proceeding. Battery characteristics expected to be improved include increases in voltage, energy density, resistance to high load, freedom of shape, and safety. Of available batteries, lithium ion batteries are the most promising secondary batteries for realizing a high voltage, a high energy density, and resistance to high load and have been and will be given improvements.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion-conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use employ a positive electrode plate prepared by applying to an aluminum current collector a mixture comprising a powdered active material, such as a lithium-cobalt complex oxide, a powdered electron conductor, and a binder resin; a negative electrode plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material and a binder resin; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
FIG. 5 schematically illustrates a cross section showing the structure of a conventional cylindrical lithium ion secondary battery disclosed in JP-A-8-83608. In FIG. 5 reference numeral 1 indicates a battery case made of stainless steel, etc. which also serves as a negative electrode terminal, and numeral 2 an electrode body put into the battery case 1. The electrode body 2 is composed of a positive electrode 3, a separator 4, and a negative electrode 5 in a rolled-up form. In order for the electrode body 2 to maintain electrical connections among the positive electrode 3, the separator 4, and the negative electrode 5, it is necessary to apply pressure thereto from outside. For this purpose, the electrode body 2 is put into a firm metal-made case to maintain all the planar contacts. In the case of rectangular batteries, an external pressing force is imposed to a bundle of strip electrodes by, for example, putting the bundle in a rectangular metal case.
That is, a contact between a positive electrode and a negative electrode in commercially available lithium ion secondary batteries has been made by using a firm case made of metal, etc. Without such a case, the electrodes would be separated, and the battery characteristics would be deteriorated due to difficulty in maintaining electrical connection between electrodes via an ion-conducting layer (separator). However, occupying a large proportion in the total weight and volume of a battery, the case causes reduction in energy density of the battery, Moreover, being rigid, it imposes limitation on battery shape, making it difficult to make a battery of arbitrary shape.
Under such circumstances, development of lithium ion secondary batteries which do not require a case has been proceeding, aiming at reductions in weight and thickness. The key to development of batteries requiring no case is how to maintain an electrical connection between each of a positive electrode and a negative electrode and an ion conducting layer (i.e., separator) interposed therebetween without adding an outer force. A method comprising bringing electrodes and a separator into intimate contact by means of a resin and the like has been proposed as a connecting means requiring no outer force.
For example, JP-A-5-159802 teaches a method in which an ion conducting solid electrolyte layer, a positive electrode, and a negative electrode are heat-bonded into an integral body by use of a thermoplastic resin binder. According to this technique, electrodes are brought into intimate contact by uniting the electrodes and an electrolyte layer into an integral body so that the electrical connection between electrodes is maintained to perform the function as a battery without applying outer force.
Being thus constituted, conventional lithium ion secondary batteries have their several problems. That is, those in which a firm case is used for ensuring intimate contacts between electrodes and a separator and electrical connections between electrodes have the problem that the case which does not participate in electricity generation occupies a large proportion in the total volume or weight of a battery, which is disadvantageous for production of batteries having a high energy density. Where the proposed method comprising bonding electrodes and an ion conductor with an adhesive resin is followed, for example, where a solid electrolyte and electrodes are merely brought into contact by an adhesive resin, the resistance to ion conduction within a battery increases due to the great resistance of the adhesive resin layer, resulting in reduction of battery characteristics.
Further, the battery according to JP-A-5-159802 supra, in which electrodes and a solid electrolyte are bonded with a binder, is disadvantageous in terms of ion conductivity as compared with, for example, batteries using a liquid electrolyte because the interface between an electrode and an electrolyte is covered with the binder. Even though an ion-conducting binder is employed, there is no binder generally known to be equal or superior in ion conductivity to a liquid electrolyte, and it has been difficult to achieve battery performance equal to that of a battery using a liquid electrolyte.
That is, a metal case is necessary for holding a liquid electrolyte in the electrode-electrolyte interface, which is disadvantageous for energy density. On the other hand, batteries of electrode-electrolyte bound type do not require a metallic case but have reduced conductivity through the electrode-electrolyte interface as compared with batteries using a liquid electrolyte, which is disadvantageous in terms of battery performance such as charge and discharge characteristics at a high load.
A nonaqueous electrolyte which can generally be used in lithium ion batteries has one-tenth or less as much conductivity as an aqueous electrolyte. Therefore, it is necessary to increase the battery area to reduce the internal resistivity. In order to make large-area electrodes into a compact battery, the electrodes are cut into strips which are laid one on top of another or the electrodes are inserted in rolled or folded band-formed separators. In practical battery assembly, a battery body is usually constructed by rolling up bands of separators and electrodes. It is possible to apply this assembly to the type of batteries in which electrodes are joined to a separator via an adhesive layer. However, the speed of rolling up the bands while applying an adhesive is lower than the speed of rolling using no adhesive, resulting in poor productivity of assembly. In a case where a battery body rolled up with no adhesive is fastened with a tape band, the internal resistivity is high because of insufficient contact at the electrode-separator interface. This is problematical in practical use particularly where a large electrical current is needed.